1. Technical Field
The present invention relates to a mode-locked laser device, a pulsed laser light source device, and a microscope device. In particular the present invention relates to a resonator length adjustable mode-locked laser device, and a pulsed laser light source device and microscope device of the same.
2. Related Art
Recently, in many industries and academic disciplines, such as optical communication and optical data processing, applications of ultra high speed optical signals in picosecond and femtosecond ranges are rapidly expanding, and demand for devices that generate such ultra high speed optical signals has increased. The most basic form of such ultra high speed optical signals is a light pulse of extremely short duration width, and it is known that if such an ultra short light pulse can first be obtained, then this can be converted into other signal forms by derivation therefrom.
Currently, as methods for generating ultra short light pulses, the method that can generate light pulses with the shortest duration width and the highest pulse repetition rate, stably, is a method of pulse generation actuation arising from a nonlinear optical effect of a laser light source itself. This type of pulse generation actuation is generally referred to as passive mode locking, or self-mode locking. In comparison to these methods there are also pulse generation actuation by application of an external modulating signal to a laser light source, these being referred to as active mode locking or forced mode locking.
In all the pulse generation actuations the pulse repetition rate of the light pulse is in an inverse relationship to the cycle time of the laser resonator. The cycle time of the laser resonator is determined by the length of the laser resonator, and the length of the laser resonator drifts with changes in temperature, or changes by fluctuating with vibrations.
Technology has therefore been proposed to avoid changes in the laser resonator length due to the surrounding temperature (see, for example, Japanese Patent Nos. 3378103 and 3450073).
When applying an ultra short light pulse generated by an light pulse laser light source device, in many cases fluctuations in the pulse repetition rate of the light pulse are undesirable. For example, when constructing a measuring system using light pulses, since the obtainable time resolution depends on the precision of pulse locking, when there is jitter due to fluctuations in the pulse repetition rate then this is detrimental to the time resolution. In such cases, the shortness of duration width corrupts and becomes meaningless. Consequently, up to now the pulse repetition rate of generated light pulses has been stabilized by controlling the resonator length in a passive mode-locked laser device.
As methods for controlling conventional resonator lengths there are methods in which one mirror configuring a resonator is fixed to a stage movable by motor driving, and a the second mirror thereof is fixed to a piezo element. The pulse repetition rate is adjusted therein by course adjustment of the resonator length by motor driving the first mirror, and then fine adjustment is performed by driving the second mirror with the piezo element. There is, for example, description in the following publication:
“Proceeding of EPAC 2006, Edinburgh, Scotland THPCH158 p. 3164”.
However, when frequency control is performed using a movable stage and piezo element, the construction of the device becomes complicated, and since driving drivers of movable stages and piezo elements are high cost, the cost of the device becomes high.